Nurturing computational thinking (CT) in students is necessary for problem solving and important for bridging the current STEM gap, such as the American K-12 students' lagging behind in math and science standardized testing. To facilitate the integration of CT in students' and teachers' experience and insights on implementing CT into K-12 classrooms, particularly elementary school classrooms, need to be investigated. We interviewed 12 teachers who had participated in a larger National Science Foundation funded STEM+C (computing) study that integrates CT in informal STEM learning guided by a project-based learning (PBL) approach. The teachers had facilitated a STEM+CT (computational thinking) curriculum as part of their professional development that introduced fourth- to sixth-grade students to CT in an afterschool community center's program over an eight-week period. We conducted a comprehensive analysis of the interviews regarding the implementation of CT into formal elementary classrooms including teachers' perceived value of CT, the challenges for classroom implementation, and their advice on how to resolve those challenges. Half of the teachers interviewed have adapted the informal STEM+CT curriculum they had facilitated in the after-school program in their own classrooms and the other half have not. We found that, while both groups of teachers cited similar challenges, teachers who have adapted the STEM+CT curriculum valued CT more specifically as a strategy for complex problem-solving. Teachers who have adapted the STEM+CT curriculum and integrated it as part of their own curriculum were more vocal and descriptive regarding the need and justifications for more teacher professional development regarding CT integration.